This invention relates to non-symmetric electrolytic/electrochemical capacitors.
A typical symmetric aluminum electrolytic capacitor (FIG. 1) includes an aluminum anode foil, an aluminum cathode foil, and a conductive liquid electrolyte, such as ethylene glycol. Ethylene glycol is a substantially non-aqueous electrolyte, i.e. it contains less than 3% of water. The liquid electrolyte is retained by a porous paper separator which acts as a spacer between the anode and cathode foils. The anode and cathode foils are connected to external terminals via aluminum tabs.
The surfaces of the aluminum anode and cathode foils are coated with a layer of an insulating aluminum oxide, which is formed by an electro-chemical oxidation process called forming. For the forming process, a constant voltage is applied to the aluminum foils. The formation voltage is higher than a typical rated working voltage of the capacitor. The aluminum oxide thickness is proportional to the applied voltage. In one example, an aluminum electrolytic capacitor may have rated working voltages up to 600 V and forming voltages in the range of 850 to 900 V.
The insulating aluminum oxide is in contact with the conductive electrolyte. The aluminum anode and cathode foils, the corresponding aluminum oxides, and the electrolyte with the separator form two capacitors connected in series (FIG. 1A). The thickness of the insulating aluminum oxide layer determines the breakdown voltage of the capacitor. By varying the aluminum oxide layer thickness, the specific capacitance (i.e., capacitance per surface area) of each capacitor is varied. Increasing the aluminum oxide layer thickness reduces the specific capacitance and increases the breakdown voltage of the capacitor. The specific capacitance may be increased by increasing the active surface area of the aluminum foil. The active surface area of the aluminum foil is increased by etching.
Another class of capacitors are the electrochemical capacitors. Electrochemical capacitors fall into two categories: Faradaic and non-Faradaic (double-layer). Non-Faradaic capacitors rely solely on interfacial charge separation across a boundary between an electrolyte and a conducting surface or an insulating surface such as some metal oxides like aluminum oxide and tantalum oxide. The Faradaic capacitors are often referred to as pseudo-capacitors. Pseudo-capacitors have enhanced charge storage derived from charge transfer through a chemical reaction that takes place across the interface between an electrolyte and a conducting surface. The charge transfer can occur, for example by: (1) surface charge attachment to a metal hydride like ruthenium hydride, (2) volume charge diffusion into a metal like silver coated palladium, or (3) an oxidation/reduction reaction at the surface of an oxide like ruthenium oxide.
Non-symmetric electrolytic/electrochemical capacitors use a conventional electrolytic capacitor at the anode and an electrochemical capacitor at the cathode. Evans U.S. Pat. No. 5,737,181 describes a non-symmetric capacitor that has a pseudo-capacitor ruthenium oxide ceramic cathode, a tantalum anode and an aqueous electrolyte. Non-symmetric capacitors with modified metal cathode surfaces are disclosed in Libby U.S. Pat. No. 4,780,797 and Rogers U.S. Pat. No. 4,523,255, which describe very aggressive aqueous electrolytes (e.g., sulfuric acid) that have high conductivity and are compatible with tantalum and tantalum oxide anodes.
In general, the invention features a capacitor of the type having a cathode and an anode and an electrolyte disposed between the cathode and the anode, the capacitor comprising an electrochemical cathode comprising a metal current collector, at least one conductive adhesion layer deposited on the metal current collector, a finely divided material deposited on the adhesion layer, an electrolytic anode comprising an oxide forming metal and a corresponding insulating metal oxide, and an electrolyte in contact with the finely divided material on the cathode and the oxide on the anode.
In preferred implementations, one or more of the following features may be incorporated. There may be a single conductive adhesion layer. The adhesion layer may comprise a carbon rubber material that provides a roughened surface onto which the finely divided material is deposited. The adhesion layer may be from 0.5 to 2.0 mil in thickness. The finely divided material may comprise carbon particles and the electrochemical cathode provides a double layer capacitance. The finely divided material may comprise a conducting metal oxide and the electrochemical cathode provides an oxidation reduction reaction. The metal oxide may comprise ruthenium oxide. The metal oxide may comprise hydrous amorphous ruthenium oxide powder. The metal oxide may be suspended in a proton conducting binder. The metal oxide and proton conducted binder may be applied suspended in a solvent, and the suspension may be applied to the metal current collector by a printing process. The anode may comprise aluminum and aluminum oxide.
Using an adhesion layer to adhere the finely divided material on the current collector produces a cathode that performs well and may be manufactured at lower cost than with prior art techniques.
Other features and advantages of the invention will be apparent from the following description of preferred embodiments, and from the claims.